Ultra-high-frequency tank circuit



Nov. 11, 1941.

T. P. KINN ULTRA-HIGH-FREQUENCY TANK CIRCUIT Filed Oct. 3, 1959 FreQu/7C] Increase WITNESSES: ,M/54am lNvENToR Theor/ore F. linn.

e Patented Nov. 1l, 1941 y UNITED STATES PATENT .OFFICE eULrRA-mGH-FmlznY-Tnnx cIRCUrr Westinghouse Electric & ManufacturingCompany, East Pittsburgh, Pa., a corporation of i Pennsylvania i v'Application October 3, 1939, Serial No. 29'l,706

l (ci. 178 44 6 Claims.

This invention relates to radio systems operating `atultra-high`frequencies and, more particularly, to circuit arrangements`for tuning such systems over'` a wide frequency range.

At frequencies lying between `60 to 200 megacyclesand higher, generallyreferred to as ultrahigh frequencies, the physical size of the reactivecomponentsis extremely critical and diiculty is experienced in thedesign and practical application of the circuits. In oscillator oramplifier tank circuits'requiring adjustmentover a certain range offrequencies, the conventionalcoil and condenser combination of paralleltuned circuits cannot be used. It is customary to employ transmissionlines for` the tank circuits either in the form of a concentricconductor or in the form of parallel `conductors. known in the art asLecher wires. These arrangements, however, have `certain physicallimitations and "when tuned either by a small size variable condenser orby a short circuiting slider, particularly applicable toV the parallelconductor arrangement, the tuning range which can be efficientlycoverediscomparatively narrow. l

, When itis necessary to cover an extended `tuning range,` for example,between 60 to ZOOmegacycles, practical investigation has shown `that ineither of the circuitsvabove referred to, to cover a tuning range of31/3 to 1, the value of the tuning capacity is so large that the ratioof inductive over capacitive reactance becomes prohibitively` low. Whenonly the inductance is varied,xto accomplish thedesired tuning, as inthe parallel line circuit, thelength ofthe conductors would have to betoo greatl considering a practical arrangement.

For example, in the parallel line circuit, with only the anode tocathode and stray capacities shunting the line, the length `of theconductors would be too great at the lower frequency portion to bepractical. Itis'customary engineering practice to'connect a capacitorbetweenv anode and cathode to: reduce this length to a` usable value.This capacitor may be a xed one, the length of the line being varied, orit may be variable to give frequency` coverage by capacitor change. Bothmay be arranged, to be simultaneously variable. 1 However, when coveringwide frequency ranges, such as 60 to 200 megacycles, the value of thecapacity which must be added to `reduce the line length to a practicalvalue at 60 megacycles, is far toolarge for efficient operation at 200megacycles. w t

The` primary :feature yof, this invention `the quency tank circuitswhile maintaining high elliciency of energy transfer throughout thetuning range.

An additional feature `of this invention is the construction of a tuningarrangement in which over the entire range of operation a substantiallyuniform ratio of inductive over capacitive reactance is maintained.

A distinct advantage of the tuning system in accordance with thisinvention is the relatively small physical dimensions necessary forconstructing altank circuit whereby it can be arranged in a small space,yet being tunable over portion of therange.

Other features and advantages will be apparent from the followingdescription of the invention, pointed out in particularity by theappended claims and taken in connection with the accompanying drawing,in which:

`Figure 1 is a schematic diagram, drawn in perspective, of a tankcircuitconnected to a pair of vacuum tubes arranged in push-pull operation;

Fig. 2 is a diagram of amodified form of the tank circuit;

Fig. 3 illustrates, by means of curves, the operation of the circuit ofFig. 1 or 2, with respect to variation of the ratio of inductive overcapacitive reactance with the increase of frequency; and

Fig. 4 illustrates the circuit losses with respect to frequencyincrease. l

Referring to the figures, the tank circuit of Fig. 1 comprises the twoparallel conductors I and 2 which are short circuited at one end by theconductor 3 to which is connected the positive side of the powersupplyindicated by the battery 4. The free terminal of conductor I isconnected to the anode 5 of the vacuum tube 6, whereas the freeterminalof the conductor 2 is connected to the anode 5 of the tube 6.The invention herein described centers around the output circuit of thearrangement shown in Fig. 1

consequently, the input circuit of the tubes beextension of the tuningrange of ultra-highr fre- 551` tween cathodesg'l and l vand grids Band 8respectively, is only schematically indicated, containing the gridresistors 9 and 9', coupling condensers ID and I and the source of biaspotential battery II. The type of input circuit may vary according todesign requirements. It may comprise, instead of the shunt feedarrangement shown here, the secondary winding of a transformer or othersuitable source of input voltage. The tuning of the output circuit iseffected by the short circuiting sliding bar I2 which may be in the formof a simple conductor or similar mechanism which shunts both conductorsI and 2 effecting thereby a short circuit at any desired point betweenthe anode terminals and the other end of the line closed by theconductor 3. At a strategic point along the line formed by conductors Iand 2, there is inserted a condenser I3 parallelling the two conductors.

The invention is illustrated and `described in connection with apush-pull type circuit, mainly because in ultra-high frequencyengineering practice, this type of circuit finds extensive application.It is understood, however, that no limitation is intended and that theinvention may as well be used with single tube circuits.

Before describing the operation of the tank circuit, reference should behad to Fig. 2 which illustrates a modification comprising the twoparallel conductors I and 2, the terminal conductor 3, and the slidingbar I2. Up to this point, the circuit is very much the same as the oneshown in Fig. 1. The modification contemplates the insertion of morethan one shunting condenser I3, such as I3', distributed at strategicpoints along the two conductors. This addition of a number of capacitiesalong the line may be accomplished by any one of a number of means,including such arrangements as specially shapedcapacitor plates fastenedto the conductors I and 2 which, in effeet, would provide any desiredvalue of capacity along the line. For the sake of simplicity, the vacuumtubes, to which the tank circuit is to be connected, have been omitted.It is to be understood that the circuit can be connected to tubes in themanner shown in Fig. 1 or can be utilized in various ways Wherever tunedcircuits of this type function as part of a radio system.

Describing the operation of the circuit, it will be seen that theshorting bar i2, when in position A, indicated in dotted lines, tunesthe circuit at the highest frequency range, let us say at 200megacycles, and it will be noted that the capacity I3 is not effectiveat this frequency because the line is, in effect, terminated by the barI2 at any position which it may occupy. Consequently, in position A fromthe anode terminal end up to and including the position when it willrest over the capacity I3, the inductive component of the circuit isvaried and the distributed capacity, together with the anode-cathodecapacity of the vacuum tube determines the capacity reactance of thecircuit. When the shorting bar I2 is placed in the position B, thecondenser I3 has become partially elective by being connected a shortdistance from the closed end of the line effected by the position B ofthe shorting bar I2. As the bar is moved in the direction of the arrow,and approachesthe end of the line determined by the conductor 3, thecondenser becomes more and more effective, since it is, in effect,progressively placed nearer and nearer the open end of the line, thatis, the anode terminal end thereof. The result is a tank circuit havinga practically.constantL/C"ratio and thecirculating current in thecircuit stays constant. As

stated before, at the high frequency end, the only capacity across theline is the tube capacity plus the very small distributed capacity whichpermits a low circulating current and high efficiency of the circuit. Atthe low frequency end, the condenser I3 is totally effective andproduces the desired circulating current with a reasonable and usablelength for the line. If a more uniform L/C ratio is desired, two or morecapacities of suitable value may be connected at proper points along theline, as shown in Fig. 2.

The curves of Fig. 3 show the advantageous operation of the circuitherein described in comparison with conventional parallel wire circuitsheretofore used. The variation of the L/C ratio is plotted along theordinate, with respect to the increase of frequency plotted along theabscissa. Curve a shows that in circuits where there is no capacitorintroduced at strategic points, the L/C ratio falls rapidly with theincrease of frequency to an impractically low value. Curve b shows theL/C constant as substantially uniform with a slight decrease at increaseof frequency and is representative of the circuit of Fig. 1, whereascurve c shows a more uniform maintenance of the L/C vratio and isrepresentative of the circuit shown in Fig. 2.

The circuit losses with respect to frequency increase are plotted inFig. 4. It is seen that in the conventional type of circuit representedby curve d, the losses rapidly increase with increase of frequency,whereas as shown by curve e illustrating the operation of Fig. 1, thereis but a slight increase of circuit loss which is within the relativepermissible design loss shown by the value X. Curve f representing theoperation of the circuit of Fig. 2, shows the possibility of maintaininga circuit loss which is well below the permissible design loss marked Xthroughout the frequency range.

The tuning system herein described permits wide frequency coveragewithout loss of efficiency aty the higher frequencies. Itmakes alsopossible the coverage of such wide range of frequencies in a restrictedspace. Practical applications of a tank circuit shown in Fig. 1 resultedin a substantially uniform circuit loss between frequencies of and 200megacycles with the following values being used. The length of theconductors I and 2 was 15 inches. The shorting bar, when placedapproximately 3 inches from the anode terminal, resonated theA circuitat 200 megacycles and when placed at the opposite end of the lineresonated at 60 megacycles. The condenser was placed approximately 5inches from the anode terminal and had a value of 70 micromicrofarads.The tubes employed were RCA type 832, the characteristics of which maybe found in the Tube Handbook Serial HB-B published by the RCAManufacturing Company.

I claim as my invention:

1. In a tuning arrangement for radio systems operating at ultra-highfrequencies and intended to cover a wide frequency range, means forchanging the resonant frequency of the circuit, and means formaintaining the ratio of inductive reactance over capacitive reactancesubstantially constant over said range, comprising means forprogressively varying the inductive reactancel over the highestfrequency portion of said range, means for automatically changing thecapacitive reactance beyond said portion and means for continuing thevariation of the inductive reactance in tuning toward the lowerfrequency end of said range. f

cuit,` connected to the output electrodes of a pair y 2,262,365 2. Ina'tunable ultra-high frequency tank` cirof vacuum tubes operated inpush-pull arrange` ment, a parallel conductor lineV forming theinductive reactance of the circuit, tuning means adapted to shortcircuit said line progressively at desired points along its lengthwhereby the eii'ec- ,y fective While said device is shunting one portiontive inductance of said tank circuit is varied,

means for maintaining the ratio of inductive over capacitive reactancesubstantially constant, in-

` cluding a Vfixed capacity interconnecting said line at a point withinthe progressive travel of said tuning means. l l

3, In a tunable tank circuit for ultra-high frequency operation, a pairof parallel" conductors interconnected at one end, tuning means for`varying the effective length of said conductors comprising aprogressively slidable contact device shunting said conductors therebyvarying the ef. fective inductance of said circuit, and a capacityinserted between said conductors in the path of movement of said device,said capacity being ineffective While said 'device is shunting oneportion of said circuit and being effective Whensaid device is shuntinganother portion.

4. In a tunable tank circuit for ultra-high frequency operation, a pairof parallel conductors interconnected at one end, tuning means forvarying the effective length of said conductors comprising aprogressively slidable contact device shunting said conductors, aplurality of fixed capacities distributed at different points in paral-Vlel with said conductors and in the path of movement of said device,said capacities being inefof said circuitand being consecutivelyeffective when said device is shunting other portions of said circuit.

v over a wide frequency range While maintaining substantially uniformratio of inductive over capacitive reactance which comprises varyingAsolely the inductive reactance at the higher frequency portion of saidrange effecting a change in capacitive reactance beyond said portionWhile varying the inductive reactance at the lower frequency of saidrange.

THEODORE P. KINN.

